In recent years, there has been widespread use in offices and homes of wireless LANs that are compliant with an IEEE (Institute of Electrical and Electronics Engineers) 802.11 standard.
Since cable connection of wireless communication devices is unnecessary in wireless LANs, wireless LANs are suited for wireless communication by battery-powered mobile wireless communication devices. There is a desire to use this type of battery-powered wireless communication device continuously for a longer period, and lowering the power consumption thereof is a major issue.
Meanwhile, there is a desire to further increase transmission speed in wireless LANs, and a method has been proposed of forming a MIMO (Multiple Input Multiple Output) channel that uses a plurality of transmission antennas and a plurality of reception antennas, and performing multiplex transmission of transmission signals via a plurality of paths by space dividing between transmission and reception terminals.
Also, another method has been proposed for further increasing the transmission speed in wireless LANs by merging a plurality of adjacent frequency channels together, thus expanding the channel bandwidth used for wireless communication.
At present, as a standard for the latter method, planning is underway for an IEEE 802.11n standard (hereinafter referred to as an “11n standard”), which is a higher-level standard than an IEEE 802.11a standard (hereinafter referred to as an “11a standard”).
The 11n standard uses the OFDM (Orthogonal Frequency Division Multiplexing) method that is the same as in the pre-existing 11a standard.
Also, the 11n standard stipulates that a frame format shown in FIG. 1B can be used for causing a wireless communication device compatible with the 11a standard (hereinafter referred to as an “11a terminal”) and a wireless communication device compatible with the 11n standard (hereinafter referred to as an “11n terminal”) to have exchangeability with each other when both the 11a terminal and the 11n terminal use the same frequency band to perform communication. The 11n standard stipulates that, in addition to the frame format of FIG. 1B (hereinafter referred to as an “11n format”), for example the frame format of FIG. 1A defined by the 11a standard (hereinafter referred to as an “11a format”) can also be used as the frame format.
Note that STS means “Short Training Symbol”, LTS means “Long Training Symbol”, SIG means “SIGNAL Symbol”, and DATA means “DATA symbol”. Also, HTSIG means “High Throughput SIGNAL Symbol”, HTSTS means “High Throughput Short Training Symbol”, and HTLTS means “High Throughput Long Training Symbol”.
In the 11n standard, when two streams are multiplexed by MIMO multiplexing and transmitted, the 11n terminal transmits, from a first antenna, a transmission signal in a frame format 50 shown in FIG. 12, and transmits a transmission signal in a frame format 60 from another antenna “CS” in FIG. 12 indicates cyclic shift. Cyclic shift refers to a known method used when transmitting an identical transmission signal from a plurality of antennas at the same time to prevent unintended beam forming by adding time lags to transmission signals in advance and transmitting the transmission signals from a plurality of antennas.
The 11n terminal adds a time lag CS1 to two transmission signals in the 11a parts (STS, LTS, SIG, HTSIG) that transmit the identical transmission signal, and performs transmission. And the 11n terminal adds a time lag CS2 that is different from the time lag CS1 to the two transmission signals in the 11n parts (HTSTS, HTLTS, DATA) that perform MIMO multiplexing, and performs transmission.
Since the time lags added to the two transmission signals are thus different between the 11a parts and the 11n parts, the characteristics of amplitude and phase of space-multiplexed signals are also different between the 11a parts and the 11n parts. For this reason, gain adjustment, frequency synchronization, symbol synchronization, etc. performed on the first of the 11a parts are not necessarily optimal in the 11n parts, and it is preferable to perform adjustments such as gain adjustment, frequency synchronization, symbol synchronization, etc. on the first of the 11n parts.
For example, to perform adjustments such as gain adjustment, frequency synchronization, symbol synchronization, etc. again on the first of the 11n parts in an environment in which both 11a format signals and 11n format signals coexist, it is necessary to decide whether a type of received signal is an 11a format signal or an 11n format signal.
As technology for performing the above-described decision, for example, there is a conventional technology for deciding whether the type of received signal is an 11a format signal or an 11n format signal by deciding whether a symbol following an SIG is an HTSIG (for example, see patent document 1).
In this conventional technology, the 11n terminal estimates a channel characteristic of the channel according to an LTS, equalizes pilot carriers included in the symbol following the LTS according to the estimated value of the channel characteristic, and reconstructs the transmission data by demapping the results of the equalization. Then, the 11n terminal decides whether the symbol following the SIG is an HTSIG by deciding whether the result of the reconstruction matches a known pattern of the pilot carrier of the HTSIG. The 11n terminal decides that the received signal is an 11n format signal if the symbol following the SIG is an HTSIG, and that the received signal is an 11a format signal if the symbol following the SIG is not an HTSIG.    Patent document 1: Japanese Published Patent Application No. 3754441.